Backlights are used to illuminate liquid crystal displays (“LCDs”). LCDs with backlights are used in small displays for cell phones and personal digital assistants (“PDAs”) as well as in large displays for computer monitors and televisions. Often, the light source for the backlight includes one or more cold cathode fluorescent lamps (“CCFLs”). The light source for the backlight can also be an incandescent light bulb, an electroluminescent panel (“ELP”), or one or more hot cathode fluorescent lamps (“HCFLs”).
The display industry is enthusiastically pursuing the use of LEDs as the light source in the backlight technology because CCFLs have many shortcomings: they do not easily ignite in cold temperatures, they require adequate idle time to ignite, and they require delicate handling. Moreover, LEDs generally have a higher ratio of light generated to power consumed than the other backlight sources. Because of this, displays with LED backlights can consume less power than other displays. LED backlighting has traditionally been used in small, inexpensive LCD panels. However, LED backlighting is becoming more common in large displays such as those used for computers and televisions. In large displays, multiple LEDs are required to provide adequate backlight for the LCD display.
Circuits for driving multiple LEDs in large displays are typically arranged with LEDs distributed in multiple strings. FIG. 1 shows an exemplary flat panel display 10 with a backlighting system having three independent strings of LEDs 1, 2 and 3. The first string of LEDs 1 includes seven LEDs 4, 5, 6, 7, 8, 9 and 11 discretely scattered across the display 10 and connected in series. The first string 1 is controlled by the drive circuit 12. The second string 2 is controlled by the drive circuit 13 and the third string 3 is controlled by the drive circuit 14. The LEDs of the LED strings 1, 2 and 3 can be connected in series by wires, traces or other connecting elements.
FIG. 2 shows another exemplary flat panel display 20 with a backlighting system having three independent strings of LEDs 21, 22 and 23. In this embodiment, the strings 21, 22 and 23 are arranged in a vertical fashion. The three strings 21, 22 and 23 are parallel to each other. The first string 21 includes seven LEDs 24, 25, 26, 27, 28, 29 and 31 connected in series, and is controlled by the drive circuit, or driver, 32. The second string 22 is controlled by the drive circuit 33 and the third string 23 is controlled by the drive circuit 34. One of ordinary skill in the art will appreciate that the LED strings can also be arranged in a horizontal fashion or in another configuration.
An important feature for displays is the ability to control the brightness. In LCDs, the brightness is controlled by changing the intensity of the backlight. The intensity of an LED, or luminosity, is a function of the current flowing through the LED. FIG. 3 shows a representative plot of luminous intensity as a function of forward current for an LED. As the current in the LED increases, the intensity of the light produced by the LED increases.
Constant current source circuits are used to generate a stable current for driving LEDs. FIG. 4 is a representation of a circuit used to generate a constant current. A constant current source is a source that maintains current at a constant level irrespective of changes in the drive voltage VDRIVE. Constant current sources are used in a wide variety of applications; the description of applications of constant current sources as used in LED arrays is only illustrative. The operational amplifier 40 of FIG. 4 has a non-inverting input 41, an inverting input 42, and an output 43. To create a constant current source, the output of the amplifier 40 may be connected to the gate of a transistor 44. The transistor 44 is shown in FIG. 4 as a field effect transistors (“FET”), but other types of transistors may be used as well. Examples of transistors include IGBTs, MOS devices, JFETs and bipolar devices. The drain of the transistor is connected to the load 45, which in FIG. 4 is an array of LEDs. The inverting input of the amplifier 40 is connected to the source of the transistor 44. The source of the transistor 44 is also connected to ground through a sensing resistor RS. When a reference voltage, is applied to the non-inverting input of the amplifier 40, the amplifier increases the output voltage until the voltage at the inverting input matches the voltage at the non-inverting input. As the voltage at the output of the amplifier 40 increases, the voltage at the gate of the transistor 44 increases. As the voltage at the gate of the transistor 44 increases, the current from the drain to the source of the transistor 44 increases. Thus, the voltage applied to the non-inverting input 42 divided by the value of RS is the constant current intended. Large displays with LED backlights use multiple constant current sources like that of FIG. 4. Therefore, large LED-backlit displays use many transistors 44.
For an LED backlit display to operate at a given brightness, the current in the drain current of the transistor 44 must be maintained at a set level: the design current. The design current may be a fixed value or it may change depending upon the brightness settings of the display.
FIG. 5 illustrates a typical relationship between the drain current and the gate voltage for an exemplary transistor. Since little to no current flows into the inverting input of the amplifier 40, the increased current passes through the sensing resistor RS. As the current across the sensing resistor RS increases, the voltage drop across the sensing resistor also increases according to Ohm's law: voltage drop (V)=current (i)*resistance (R). This process continues until the voltage at the inverting input of the amplifier 40 equals the voltage at the non-inverting input. If, however, the voltage at the inverting input is higher than that at the non-inverting input, the voltage at the output of the amplifier 40 decreases. That in turn decreases the source voltage of the transistor 44 and hence decreases the current that passes from the drain to the source of the transistor 44. Therefore, the circuit of FIG. 4 keeps the voltage at the inverting input and the source side of the transistor 44 equal to the voltage applied to the non-inverting input of the amplifier 40 irrespective of changes in the drive voltage VDRIVE.
However, if the drive voltage VDRIVE falls too much, the low VDRIVE causes the transistor to operate in the triode region and lose regulation. In the triode region of operation, the transistor behaves as a resistor and the drain-source current is largely dependant on the drain-source voltage instead of the gate-source voltage. The transconductance gm of the transistor is given by the following equation:
            g      m        =                  ∂                  I          DS                            ∂                  V          GS                      ,where IDS is the drain-source current of the transistor and VGS is the gate-source voltage of the transistor. In the saturation region, the drain-source current IDS is given by the following equation: IDS=K(VGS−Vtr)2, where Vtr is the threshold voltage of the transistor and K is a constant associated with the transistor. In the triode region, the drain-source current IDS is given by the following equation: IDS=K′(2VGSVDS−VtrVS−VDS2), where VDS is the drain-source voltage of the transistor and K′ is a constant associated with the transistor. As the above equations illustrate, the drain-source current is proportional to the square of the gate-source voltage in the saturation region and is proportional to the first order gate-source voltage in the triode region. Further, the transconductance of the transistor is proportional to the first order gate-source voltage in the saturation region. But in the triode region, the transconductance is constant relative to the gate-source voltage and is proportional to the drain-source voltage.
FIG. 6 shows an exemplary relationship between drain-source current and drain-source voltage in a transistor for various gate-source voltages VGS1, VGS2, VGS3, VGS4. In the region to the right of the triode transition voltage 61, 62, 63, 64, the drain-source current does not vary significantly with changes in the drain-source voltage. This region is known as the saturation region of the transistor. In the region to the left of the triode transition voltage 61, 62, 63, 64, the drain-source current varies significantly with changes in the drain-source voltage. This region is known as the triode region.
One way of avoiding operation in the triode region is to design a constant current circuit with sufficient overhead such that the typical drain-source current of the transistor can fluctuate significantly without entering the triode region. However, since the power dissipated in a transistor is equal to the product of the drain-source current IDS and the drain-source voltage VDS, this method results in increased power dissipation across the transistor and may require a larger-footprint transistor.